MEP018 shorted windings..how?

[cranetruck]

Having made the unpleasant discovery that my 10KW generator suffer from a short circuit between two of its windings, T2, T5 to T8, T11, I wondering how this can happen. Perhaps some of the electrical heavyweights, Isac, Speedemon and others can help explain.

This equipment is extremely well built, IMHO and it looks like fixing this is beyond what I can do here at home.

If there is a parts MEP018 somewhere, let me know.

Does someone have a link to the TM 5-764 manual? I'd like to know how the connections are made to the connector in the back of the unit, can the connector be pulled out for inspection? The diagrams I have don't show how the stator windings are wired to the connector...would be nice if the short was in that part somewhere...
At least, the 4A084 starts easily and runs well now.
TIA

 

[PeterD]

Classic failures like this (and this applies to motors, too) are old insulation failing, physical damage, and sometimes (though rare) damage from things such as lightning.

Overloading (and attendant overheating) will hasten failures (by causing the wire's insulation to deteriorate more quickly.)

Sometimes it is worth while to check where the winding wires are connected to the wires used to bring power out of the generator head, sometimes the failures are there and that can be easy to fix.

 

[cranetruck]

....
Sometimes it is worth while to check where the winding wires are connected to the wires used to bring power out of the generator head, sometimes the failures are there and that can be easy to fix.

That's one solution I am hoping for, but need to know how much the connector can be pulled out safely. It's pretty tight and will only come out about 1/2 inch or so and I'm not ready to pull harder without knowing what's inside.

 

[PeterD]

AFAIK, (Don't have that gen head to see) you'd have to disassemble it to get at the splices. Generally they are 'wrapped' with the windings themselves. So basically complete disassembly, and go from there. Face it, if it is bad, you can't do any more damage to it taking it apart, and you may be able to fix it.

 

[Isaac-1]

If I am reading this right T5,T2,T8, and T11 are all showing continuity to each other? If so these would all be on L2 of a 120/208 low wye 3 phase connection, my guess is someone tried to pull too much load off one leg and overheated the windings at some point which may have causes a delayed failure of the insulation. Having said this since they are all on one leg of 120/208, might be able to isolate those leads and still use 2 legs to generator 120/208V in place of 120/240 split phase for household backup. Most older voltage regulators only sense the voltage on a single leg of 3 phase, usually L1 so that side of things should not be hard to deal with.

Ike

 

[Carl_in_NH]

If all you've got is an interwinding short, then what Ike suggests is worth investigating. If it ended up with a shorted turn or two in addition to the interwinding short then you're done - and that one is hard to detect with an Ohm meter. It's worthwhile to tear into it and look for heat related damage.

 

[Speddmon]

Heat damage might be easy to take a quick peak at...the insulating varnish used doesn't like heat. Especially the older varnish. If it got hot enough to short a winding, try taking a look-see with a flashlight at the coils if you can get to a point where you can look into a crack in the housings. If the varnish looks darkened and discolored, or looks like it got hot enough to partially liquify and drip, more than likely you have some major heat damage.

If you can't take it apart or you don't want to try to get into it that far, try to find a motor re-wind shop close by and see if they could fix it without putting you in the poorhouse.

 

[PeterD]

I doubt it is just an inter-winding short, IIRC, the OP did a test with the head disconnected and the problem continued... So I suspect that won't help. I like Speddmon's comment about looking for heat damage too, but I'll go a bit further and suggest that a disassembly is the best way to go. It is still possible that the shorts are relatively external (at the lead splices) and if so it may be repairable.

Then again, it may turn out that the short is deep in the coils, and only a rewind (or replacement...) is the only viable fix. The MEP-004's manual covers rewinding the head, though it is not a job for those who have never done rewinding of motors...

 

[cranetruck]

If I am reading this right T5,T2,T8, and T11 are all showing continuity to each other? If so these would all be on L2 of a 120/208 low wye 3 phase connection, my guess is someone tried to pull too much load off one leg and overheated the windings at some point which may have causes a delayed failure of the insulation. Having said this since they are all on one leg of 120/208, might be able to isolate those leads and still use 2 legs to generator 120/208V in place of 120/240 split phase for household backup. Most older voltage regulators only sense the voltage on a single leg of 3 phase, usually L1 so that side of things should not be hard to deal with.

Ike

The most severe load that the generator has seen during the last 25 hours of operation (since last July) would be the water heater, which is somewhere around 4KW. Don't know what is considered an overload with the "split phase" hook-up.

Interesting idea rewiring so that the shorted windings are no longer loaded down...I have eliminated the voltage/phase selector switch and can make all possible connections with the windings terminated on the terminal blocks as shown in the image below.
The diagrams show the present connections and the shorted windings.

So, what are the possibilities?

 

[Isaac-1]

My thought was to wire it in low wye 120/208 3 phase with the all the components of the L-2 leg isolated (not tied together). In a standard 12 lead this would be:

T5,T2,T8,T11 isolated

T4,T6,T10,T12 tied together for L0 neutral

T1,T7 tied together for one of the Line outputs, designate L1 normally

T3,T9 tied together for one of the Line outputs, designate L3 normally, but in this case you would connect it to a single phase connection as L2

Ike

p.s. check for metering lines tied in ahead of this point.

 

[cranetruck]

My thought was to wire it in low wye 120/208 3 phase with the all the components of the L-2 leg isolated (not tied together). In a standard 12 lead this would be:

T5,T2,T8,T11 isolated

T4,T6,T10,T12 tied together for L0 neutral

T1,T7 tied together for one of the Line outputs, designate L1 normally

T3,T9 tied together for one of the Line outputs, designate L3 normally, but in this case you would connect it to a single phase connection as L2

Ike

p.s. check for metering lines tied in ahead of this point.

Thanks Ike!
So I'll end up with this and the VR and the convenience outlet rewired to pic up a new 120 VAC leg (L2 to L0 or L1 to L0).

 

[Isaac-1]

That looks right from what I can see, but I can't make out all the T numbers on your attachment.

Ike

p.s. I am not sure if the voltage regulator on this model uses a 120V or 208-230V reference if 208-230V then you would likely need to rewire it between the two active Lines as the most likely default is original L1-L2, otherwise it would be as you noted for 120V on the sense leads

 

[cranetruck]

That looks right from what I can see, but I can't make out all the T numbers on your attachment.

Ike

p.s. I am not sure if the voltage regulator on this model uses a 120V or 208-230V reference if 208-230V then you would likely need to rewire it between the two active Lines as the most likely default is original L1-L2, otherwise it would be as you noted for 120V on the sense leads

An enlarged portion of the diagram attached.

By the wiring diagram, the VR works on 120VAC (same as convenience outlet).

I plan on working on this tomorrow. Your help is greatly appreciated!

 

[cranetruck]

Alright, so what are the consequences of this modified "120/208V 3 phase" wiring modification?
Image below shows the wiring according to the selector switch on top (ideal) and the modified wiring as suggested by Ike with the shorted windings isolated.
Power capacity? Is 208VAC harmful to appliances?
Where does the 208VAC come from, why not 240 if there is 120 across each leg, phase relationship, probably? Time to go back to the old text books, I guess...or a quick explanation would be appreciated.

 

[Carl_in_NH]

You are correct, Bjorn - it's the phase relationship between the two windings that results in less than 2x voltage.

Some loads are rated for this lower voltage, some are not; check your loads to find out. Motors are of prime concern ( as are some transformer powered electronics - but you don't typically have much of that in the home that runs off 240V) because they will run hotter at lower voltage, but things like hot water heaters will be just fine.

 

[Isaac-1]

Yes, check your data plates, if a motor is marked 208-230V your ok, it is marked 230V it may over heat, however you may get away with it by turning up the voltage some to around 126/228V or a little less, the general spec is +/- 10% allowed for voltage with a nominal of 115V on the low side. Many appliances are marketed to be sold to both homes and light commercial area with 120/208V 3 phase, some things like water heaters may be marked for both or just 230V (I suspect it depends if the brand sells a separate more expensive 208V model), as long as it only has a heating element it will be ok, just not get as hot and draw less amps, same for electric stove if you were to try to run one on 208V. Other than motors you have to watch out for old 24VAC transformers powered from the 230V line iin the control system of central heating units and air conditioners, newer units tend to have switch mode power supplies powered from the 120V line.

Ike

 

[cranetruck]

Thanks guys, and what about current capacity with one less set of windings or doesn't that come into play?

 

[PeterD]

As far as the shorted windings go... If the coils are internally shorted, then it won't matter what you disconnect--it still will cause problems. The reason for this is that a single shorted turn attempts to draw all available current (it behaves like a coil with a short on it!) Of course the wire size is too small to allow all current to flow, so the wire heats up, eventually leading to release of the magic smoke that we're all familiar with.

A single winding to winding short might allow you to drop off one of the legs and still have it work, but IIRC, the OP tried running it with all disconnected and it was still bad (maybe I'm wrong on this, and he'll clarify that for me.)

 

[Isaac-1]

Peter you do have a point, but it can't hurt to try and see if it works at this point, it just depends on the details of the failure now that the ends are also isolated..

Your safe output will be reduced to 2/3 of the rating since your using 2/3 of the windings, in reality the rating is reduced to about 70% or so due to the unused winding still radiating heat.

Ike

 

[cranetruck]

Thank you Peter and Ike for the heads up on the possibility of heat/smoke...
I'll have a thermocouple probe taped to the generator case, so that any radical temp increase can be caught in time.
The control box is now rewired for the 120/208 3ph (2 ph?) as shown above. I had everything ready to go about 1/2 hour ago, but need to get more gas and will run it tomorrow.
The shorted windings are terminated on a terminal block, all open ended.